Transducers, such as those employed in the medical ultrasound imaging industry, typically employ piezoelectric ceramic materials such as lead-zirconate-titanate (PZT) to both emit and receive ultrasound waves. For various reasons, including enhanced formability and acoustical performance, it can be advantageous to use a piezoelectric composite rather than a monolithic block of PZT.
Composite piezoelectric transducers, and methods for their production, are described, for example, in Composite Piezoelectric Transducers; R. E. Newnham et al; Materials in Engineering, Vol. 2, December 1980, pages 93-106, which is incorporated herein by reference.
Piezoelectric composites typically consist of an polarizable phase embedded in a non-polarizable material. These composites have many advantages over traditional monolithic piezoelectric ceramics including: (i) lower densities resulting in acoustic impedances closer to those of the human body, water, etc., eliminating the need for an acoustic matching layer, (ii) low dielectric constants, resulting in high piezoelectric voltage constant g, and (iii) ease of conformability to the shape of the backing material of the composite.
Two composite designs that have been particularly successful are composites with 2--2 and 1-3 connectivity, in which piezoelectric ceramic sheets or rods are aligned in the poling direction of the composite (in a matrix of a suitable polymer.) In the case of the 2--2 composite, both the ceramic and polymer phases are two-dimensionally connected throughout the composite. The stiff ceramic phase supports most of the stress applied in the direction of its alignment, yielding a high piezoelectric charge coefficient d, while the composite has a low density and dielectric constant.
In the 1-3 composite, the ceramic phase is one-dimensionally connected through the composite, while the polymer phase is three-dimensionally connected. The 1-3 composite yields superior properties to those described above for the 2--2 composite due to the lower density and lower dielectric constant.
A common and convenient method for making 2--2 and 1-3 composites is to start by cutting parallel slots into a monolithic piezoelectric ceramic block. The slots are then filled with a polymer. Typically the polymer used is non-polarizable. If one set of parallel slots are cut, the resulting composite has a 2--2 connectivity. If a second set of parallel slots are cut orthogonally to the first set of slots and backfilled with a filler material, the resulting composite has a 1-3 connectivity. Composite fabrication is completed by grinding the unslotted section of the monolithic ceramic away from the slotted section lapping the two sides parallel to one another, and electroding the surfaces containing exposed ceramic. The resulting structure essentially comprises a semiflexible mat consisting of strips, posts or rods of piezoelectric material laterally encased by polymeric matrix material such as epoxy.
A strong electric field is then applied to the composite in a direction perpendicular to the plane of the sheet using conventional poling or corona discharge techniques. The intensity of the electric field used will ordinarily be selected to provide efficient polarization. However, it will be kept below the range at which substantial dielectric breakdown of the material being polarized occurs. The aforementioned method is known as the "dice and fill" method and is described in PZT-Epoxy Piezoelectric Transducers: A Simplified Fabrication Procedures, H. P. Savakus et al; Materials Research Bulletin, Vol. 16, 1981, pages 677-680, which is incorporated herein by reference.
Although, as mentioned above, piezoelectric composites typically consist of a polarizable phase embedded in an non-polarizable phase, there is a need to development efficient methods for the manufacture of composites having multiple polarizable and/or non-polarizable phases. Finally, there is a need to develop more efficient methods for the manufacture of piezoelectric composites having decreased size and periodicity of the polarizable phase or phases. Composites having these properties have been identified as a key area of transducer development.
Some examples of these needs are evidenced in the field of medical imaging. The resolution of imaging systems can be enhanced by increasing the frequency of the generated sound wave. To increase the frequency of the sound wave, the size and periodicity of the polarizable ceramic phase in the composite must be decreased. In addition, enhanced pulse-echo properties are realized if the transducer contains transmitting and receiving sections wherein the transmitting sections contain an polarizable phase with high transmitting sensitivity, while the receiving sections contain an polarizable phase with high receiving sensitivity.
Moreover, there has been a drive to create so-called "smart" materials. Smart materials are described in "Smart Ceramics"; Newnham et al., Ferroelectrics, Vol. 102, pp. 259-266 which is incorporated herein by reference. A smart material senses a change in the environment, and using a feedback system, makes a useful response. It is both a sensor and an actuator. A very smart material can tune its sensor and actuator functions in time and space to optimize behavior. Tuning of a very smart material can be accomplished by using a multitude of polarizable phases.
Finally, it would be desirable to have an efficient method to tailor the piezoelectric properties within a single transducer by varying the ceramic volume content across the device. This reduces the out of plane distortions of the transmitted signal.
The "dice and fill" method has several limitations in meeting the above stated needs. Dicing technology utilizes thin, diamond coated blades rotating at high speeds. The blades, often thinner than 200 microns, are expensive, difficult to handle, and have short usage lives. Moreover, the "dice and fill" technique uses monolithic ceramics, ruling out the possibility of composites with multiple polarizable ceramic phases. Finally, varying the ceramic volume content across the transducer would require frequent changes of dicing blade to vary the thickness of the slots, substantially adding to the time it takes to produce a transducer.
Alternatives to the "dice and fill" technique have attempted to address one or more of the above issues. Diepers (U.S. Pat. No. 4,564,980) and Zola (U.S. Pat. No. 4,572,981) teach the formation of fine 1-3 composites by bonding alternating layers of active and passive materials to form a laminated block. The laminated block is sliced to yield composites with 2--2 connectivity. The 2--2 composites are then bonded with a passive material to yield 1-3 composites. The present invention is advantageous over both Zola and Diepers in that there is no need to restack the composite to form 1-3 connectivity. This results in a process with much greater efficiency than both Zola and Diepers.
'T Hoen (U.S. Pat. No. 4,518,889) teaches the formation of a 1-3 composite with a varying ceramic volume content across it. Ceramic rods were aligned parallel a polymer matrix using positioning plates with a plurality of holes. The density of holes in the plate was a function of radial plate location so that there were more rods near the center of the plate than near the edges. This technique is severely limited by the time consuming and inefficient process of placing the rods in the positioning plates. Moreover, as the diameter of the rods decreases below 50 microns, composite manufacturing becomes impractical. The present invention is thus a more efficient and practical method for the manufacture of volume gradient piezoelectric composites.